This invention relates to an ignition system of an internal combustion engine, and more particularly to a battery type ignition system.
Depending upon the state of an ignition transformer at the time of ignition, the ignition system of the type referred to above is generally classified into current interruption type and voltage impression type, and the former type is now used extensively.
In one construction of the ignition system of this type, the primary winding of an ignition transformer and the contact of an interrupter are connected in series with a source of direct current, and an ignition plug is connected across the secondary winding of the ignition transformer. Upon closure of the interrupter contact, primary current that increases gradually at a rate determined by the inductance and the DC resistance component of the primary winding flows therethrough to store electromagnetic energy in the ignition transformer. When the interrupter contact is opened the primary current is interrupted with the result that the magnetic flux in the core of the ignition transformer decreases rapidly due to the rapid decrease in the primary current. Due to electromagnetic induction caused by this rapid variation in the flux, a high voltage is induced in the secondary winding which strikes a spark across the ignition plug. While the construction described above is relatively simple and inexpensive, it has the following disadvantages. More particularly, since in the current interruption type it is necessary to store a large electromagnetic energy in the transformer for generating high voltage, it is usual to use an open core type ignition transformer. Although this type of the ignition transformer has a large reluctance and is difficult to saturate magnetically so that it is suitable to store a large magnetic energy, it is difficult to design it to have a large effective permeability so that it is necessary to increase the number of turns of the winding in order to obtain a required inductance. As a consequence, not only the loss is large and the efficiency is low but also the secondary distributed capacity, the secondary DC resistance component and the leakage reactance are increased. This results in the increase of the output impedance and in the poor build up characteristic of the secondary voltage. Furthermore, the open core type ignition transformer can not produce high secondary voltage when the leakage resistance decreases due to deposition of carbon on the ignition plug.
One example of the voltage impression type ignition system is disclosed in U.S. Pat. No. 3,318,295 having a circuit construction as shown in FIG. 1 of the accompanying drawing. As shown, a DC source, for example a battery 11, is connected to a DC-DC converter 12 which boosts DC voltage. A primary winding 15a of an ignition transformer 15 and a semiconductor controlled rectifier or thyristor 16 are connected in series and they are connected in series with the converter 12. A capacitor 14 is connected in parallel with series connection of the primary winding 15a and the thyristor 16. The secondary winding 15b of the ignition transformer 15 is connected across an ignition plug 17. The gate electrode of the thyristor 16 is connected to a trigger circuit 18 which generates a trigger pulse for operating the plug 17 at a desired ignition angle.
In the operation of the circuit shown in FIG. 1, the voltage of the DC source is boosted by the DC-DC converter 12 for storing an electrostatic energy in capacitor 14. When a trigger pulse is applied across the gate and cathode electrodes of the thyristor 16 from the trigger circuit 18, the thyristor 16 is rendered conductive to discharge the energy stored in the capacitor through the primary winding 15a of the ignition transformer 15. In other words, the voltage across the capacitor 14 is impressed across the primary winding 15a. Consequently, high secondary voltage is induced across the secondary winding 15b which is applied across the ignition plug 17 thereby igniting a fuel-air mixture.
With this construction, the ignition transformer 15 is not required to store electromagnetic energy as in the current interruption type so that it is possible to use a closed core type ignition transformer. The closed core type ignition transformer has a small reluctance and loss as well as a high efficiency so that it is possible to make it smaller than the open core type ignition transformer. Moreover, since the output impedance is low it is possible to produce high secondary voltage. Further, since its secondary distributed capacitance is small, its secondary voltage build up characteristic is greatly improved. For this reason, even though the ignition plug is contaminated more or less by the deposited carbon, it is possible to produce sufficiently high secondary voltage. In this manner, the closed core type ignition transformer can eliminate most of the defects of the open core type ignition transformer, but it is impossible to obtain a high voltage without boosting the voltage of a DC source, such as a battery. Thus, this type of ignition system requires to use a DC-DC converter thus increasing the number of component elements and the cost of the ignition system.